Frequency conversion through the use of electronic switches is widely known, and typical electronic switches have been spark gaps, thyratrons, vacuum tubes, transistors and thyristors. At the present time, thyristors are preferred for generating high frequency power up to 10 kHz due to their unlimited life, low cost and high energy switching capacity per switch.
A thyristor, or silicon controlled rectifier (SCR) will conduct in response to a control pulse when it is forward biased and will continue conducting until a reverse voltage is applied across the SCR. Systems employing SCR's in the kHz range are most often resonant circuits, wherein the requirement of the SCR to have a reverse voltage applied for approximately 10 to 60 microseconds in order to turn it off is met by the natural reversal of the current in the resonant circuit. A common circuit which operates in this manner is the series inverter shown in FIG. 1, wherein only the main power components are shown. When SCR's 10 and 12 are switched on, the series circuit consisting of the capacitor 14 and transformer primary 16 having a substantial leakage reactance is resonantly charged to approximately 1.5 to 2.0 times the source voltage. The capacitor then causes current to flow back to the source via diodes 18 and 20, thus causing SCR's 10 and 12 to turn off. When SCR's 10 and 12 have turned off, SCR's 22 and 24 are fired.
With a circuit operating in this manner, the turn off time available to the SCR's is only a fraction of one-half cycle of operation and, even if only a 10 microsecond turn off time is required, it is difficult to achieve frequencies in excess of 10 kHz.
Some variations of the circuit of FIG. 1, such as replacing diodes 18, 20, 26 and 28 with resonant LC networks, make it possible to turn off the thyristors with a reverse voltage which lasts longer than a half cycle of the resonant load, but the resonant load decays in amplitude before it can again be energized via SCR's 22 and 24. Induction heating devices preferably utilize 50 kHz power and, in such a case, the load would be a 50 kHz resonant circuit. If this resonant circuit is only supplied with a pulse of energy at a 10 kHz repetition rate, the voltage of the load will decay between pulses and optimum heating cannot be achieved.
A further approach to obtaining high frequency power is the use of a shunt commutated impulse circuit as shown in FIG. 2. In the circuit of FIG. 2, SCR's 30 and 32 conduct and charge the commutating capacitor 34. When a predetermined voltage is reached, the output SCR 36 is fired and the current source 38 feeds the load. At a subsequent time, SCR's 40 and 42 can be fired to connect the charged capacitor inversely in the circuit, thus reverse biasing the output SCR and terminating the pulse supplied to the load.
With the circuitry of FIG. 2, an output pulse can be achieved which is narrow enough to excite a high frequency tank circuit. However, a relatively long time must elapse while the commutating capacitor 34 charges up before the circuit can deliver another pulse and, therefore, the amplitude in the resonant load circuit will decay between pulses. By connecting a number of the FIG. 2 circuits in parallel, the tank load can be excited at the proper frequency, but each pulse will require a separate current source, separate output SCR and separate commutating circuit. This becomes excessively costly.
An additional problem with the circuit of FIG. 2 is that the commutating capacitor will only provide turn-off time to the output SCR as long as it is more negative than the tank circuit, but the tank circuit is ringing both positive and negative. As a result, lower operating voltages must be used in order to ensure sufficient turn-off time.
A further technique for generating high frequency power is disclosed in U.S. Pat. No. 3,290,581 to Hooper. As shown in FIG. 3 of that patent, a plurality of SCR diamond commutating circuits are used to achieve a type of frequency multiplication. Half of the diamond circuits are connected in parallel to one side of a transformer primary while the other half are connected in parallel to the other side of the transformer primary and the firing order of the diamond circuits can be shifted in phase so as to assist alternate and opposite directions of current flow through the primary. While the Hooper circuitry is an improvement over the previously discussed frequency generation circuits, it has still not proven entirely satisfactory. Turn off time is assured by virtue of firing alternate pairs of cross corner thyristors in a given diamond. If the cross corner pair is not immediately fired, the initial thyristor current will go to zero since the capacitor cannot conduct d.c. current. Turn off pulses are also supplied by virtue of adjacent diamonds, and after the adjacent diamonds have fired, the firing order returns to the initial diamond and the opposite cross corner pair is fired, thereby providing an additional turn off pulse to the initial pair considered. Thus, turn off time is no longer the limiting factor in the Hooper system. However, the diamonds and their capacitors are not used in a continuous current mode, nor are the primary windings of the transformer. Tuning out of inductance inherent in the construction of the equipment is not possible in discontinuous current. The Hooper system also employs a voltage source to supply energy to the diamond arrays and does not enjoy the protective advantages of being supplied with a current source, to be described.
Canadian Pat. No. 1,079,363 discloses a high frequency power generation circuit utilizing a plurality of diamond bridge circuits connected in parallel with one another and also in parallel with a load. While this may be preferable to the high frequency voltage generating circuits discussed above, it is still not entirely satisfactory at very high frequencies and high power, e.g. on the order of 100 kHz and 100 kW. Each diamond bridge circuit includes a commutating capacitor which must be repeatedly charged and discharged at currents well in excess of 1000 amperes, and at very high frequencies this charging and discharging may encounter substantial impedance from the panel inductance which is inherent in each of the diamond bridge circuits. Thus, with the circuit configuration illustrated in Canadian Pat. No. 1,079,363, the individual diamonds are not operating in continuous current, and the inductive impedance within the diamond in series with each capacitor cannot be tuned out. During conduction, voltage from the capacitor is lost in overcoming this inductive inertia, which is significant when only 5 or 10.mu.seconds are available for delivering a high energy pulse. This situation is aggravated by the fact that thyristors are relatively high current low voltage devices, i.e. 1000 AMPS, 600 volts compared to vacuum tubes which may operate at 10,000 volts or higher. Thus, stray inductance which stores energy as Li.sub.2 is a greater problem at high current and low voltage.